CD28 is expressed on most T lineage cells and plasma cells (June, C. H. et al., Immunol. Today 11, 211-16 (1990); Damle et al., Proc. Natl. Acad. Sci. 78:5096-6001 (1981)). The ligand for CD28 is B7, which is expressed on activated B cells (Linsley, P. S. et al., Proc. Natl. Acad. Sci. USA 87, 5031-35 (1990); Linsley, P. S. et al., J. Exp. Med. 173, 721-730 (1991).
CD40 is a member of the tumor necrosis factor receptor (TNFR) family of type I membrane-bound signaling receptors. Though originally identified as a B cell antigen, CD40 is expressed by all antigen presenting cells (APC) including dendritic cells, monocytes, and B cells.
The ligand for CD40 is gp39, which binds to CD40 and thus can activate B cells. Gp39 is also known as CD40L, TRAP and T-BAM. Gp39 is a type II cell surface protein with significant homology to TNF and is transiently expressed by activated T cells. In addition to T cells, gp39 is expressed by basophils, mast cells, and eosinophils.
The CD28 and CD40 pathways play essential roles in the initiation and amplification of T-dependent immune responses (Bluestone, J. A. Immunity 2, 555-9 (1995); Banchereau J., et al. Ann. Rev. Immunol. 12, 881-922 (1994); Durie, F. H., et al. Science 261, 1328-30 (1993); Foy, T. M., et al. J Exp Med 178, 1567-75 (1993); Van den Eertwegh, A. J. M., et al. J Exp Med 178, 1555-65 (1993)).
CD28/B7 interactions provide critical "second signals" necessary for optimal T cell activation, and IL-2 production (Jenkins, M. K., et al. J. Immunol. 147, 2461-6 (1991); Schwartz, R. H. Cell 71, 1065-8 (1992); Boussiotis, V. A., et al. J. Exp. Med. 178, 1753-1763 (1993)), whereas CD40/gp39 signals provide costimulation for B cell, macrophage, endothelial cell, and T cell activation (Grewal, I. S., et al. Nature 378, 617-620 (1995); van Essen, D., et al. Nature 378, 620-623 (1995); Hollenbaugh, D., et al. J. Exp. Med. 182, 33-40 (1995); Armitage, R. J., et al. Nature 357, 80-2 (1992); Cayabyab, M., et al. J. Immunol. 152, 1523-31 (1994); Noelle, R., et al. Proc. Natl. Acad. Sci. USA 89, 6550-6554 (1992); Alderson, M. et al. J. Exp. Med. 178, 669-674 (1993)).
Host immune responses often cause rejection of transplanted tissues and organs. Thus, inhibition of those immune responses are critical in the success of tissue transplantation. There have been studies aimed at blocking either of the CD28 or CD40 pathways, however, blockade of either of these pathways alone has not been sufficient to permit engraftment of highly immunogenic allografts (Turka, L. A., et al. Proc. Nat'l Acad. Sci. USA 89, 11102-11105 (1992); Parker, D. C., et al. Proc. Nat'l Acad. Sci. USA 92, 9560-9564 (1995); Larsen, C. P. et al. Transplantation 61, 4-9 (1996)). The monotherapies blocking either CD28 or CD40 pathway only resulted in at best temporary, and sometimes longer, periods of survival of transplanted tissues. Neither blockade alone uniformly promoted graft survival.
The vigorous immune response to xenogeneic organ transplants has served as a powerful barrier to the application of this technique to clinical transplantation (Platt J. L., Curr. Opin. Imm. 8, 721-728 (1996). Previous experimental attempts to prolong xenogeneic skin grafts have required either whole body irradiation followed by mixed xeno/syngeneic reconstitution (Ildstad S. T., Sachs D. H., Nature, 307: 168-170 (1984)), or rigorous preconditioning with thymectomy combined with depleting anti-T cell antibodies (Pierson III R. N., Winn H. J., Russell P. S., Auchincloss Jr. H., J. Exp. Med., 170:991-996 (1989); and Sharabi Y, Aksentijevich I., Sundt III T. M., Sachs D. H., Sykes M., J. Exp. Med., 172:195-202 (1990). These strategies have recently been used to promote skin graft acceptance across a discordant xenogeneic barrier (Zhao Y., Swenson K., Sergio J., Arn J. S., Sachs D. H., Sykes M., Nat. Med., 2(11):1211-1216 (1996)). However, the potential morbidity associated with cytoablative treatment regimens present a significant obstacle to the introduction of these strategies into use in clinical solid organ transplantation. Thus the development of non-cytoablative strategies to prolong xenograft survival would greatly facilitate the clinical application of these techniques.
Presently, there exists a need to provide ways to effect long-term tolerance of transplanted tissues by the host, thereby increasing the survival rate of transplantation. To do so, it is necessary to ensure sufficient immunologic unresponsiveness in the transplant recipient.
We have found that the inhibition of T-dependent immune responses resulting from blockade of either CD28 or CD40 signals is potent, but incomplete. The data herein demonstrate that simultaneous blockade of these pathways unexpectedly inhibits acute and chronic rejection of transplanted tissue in vivo. Independent blockade of these pathways using a soluble CTLA4 molecule or antibodies which recognize and bind gp39 failed to even minimally prolong survival of primary skin transplanted tissue.
The invention herein involves the discovery that simultaneous blockade of CD28 and CD40 signals promoted long-term survival of fully allogeneic as well as xenogeneic skin grafts. Prolongation of skin allograft survival was eliminated by cyclosporine A (CyA), suggesting that it is an active process requiring intact signaling via the TcR/CD3 complex and/or other CyA sensitive pathways. Moreover, CTLA4Ig/MR1 promoted long-term acceptance of primarily vascularized cardiac graft tissue, and inhibited the development of chronic vascular rejection.
The effect demonstrated in the two transplantation models herein indicates that CD28 and CD40 provide interrelated, yet independent signaling pathways required for the generation of effective T cell responses. This discovery provides methods which are new and more effective strategies to manipulate immune responses including suppressing graft rejection.